In a spread spectrum radio system, and particularly in a CDMA technique, the narrowband data signal of the user is modulated by a spreading code of much wider bandwidth to a relatively wide band. In the known experimental systems, the bandwidths used include, for example, 1.25 MHz, 10 MHz and 50 MHz. The spreading code usually consists of a long pseudo-random sequence of bits. The bit rate of the spreading code is much higher than that of the data signal, and to make a distinction between data bits and symbols, the bits of the spreading code are referred to as chips. Each data symbol of a user is multiplied by all the spreading code chips. Consequently, the narrowband data signal spreads to the frequency band used by the spreading code. The signal of each user has a separate spreading code. Several users transmit their signals simultaneously by using the same frequency band, and the signals from the users can be identified from one another in the receivers on the basis of the pseudo-random spreading code.
Regardless of the efficient interference and fading protection based on spread spectrum technology, the connection quality, affected by the signal and noise quality, between a base station and a mobile station in the radio system constantly varies. The variation results from disturbance present on the radio path and the attenuation of radio waves as a function of distance and time in the channel fading out. The connection quality can be measured for example by monitoring the strength level received. In order to partly compensate for the quality variations of the connection, power control can be used.
A digital radio system utilizing a multilevel orthogonal modulation such as a Walsh-Hadamard transform requires a more accurate method than power measurement in order to estimate the connection quality. In addition, it is often important to know whether the receiver is picking up a signal or just noise as it is not worthwhile to apply noise to the detector of the receiver. Nevertheless, the amplitude variation of the noise is a useful piece of information. The known parameters for quality of the connection include Bit Error Rate (BER) and signal to noise ratio (SNR).
It is known to utilize the decisions of Viterbi detection in estimating the SNR of the received signal. Due to the Viterbi algorithm often being too demanding a task for a digital signal processing program, separate Viterbi hardware has to be used. This is described in greater detail in J. Hagenauer, P. Hoeher: A Viterbi Algorithm with Soft-decision Outputs and its Applications, IEEE GLOBECOM 1989, Dallas, Tex., November 1989, which is herein incorporated as reference.
As known, the SNR information is needed when using different kinds of diversity receivers in which a signal received from different diversity branches is combined. For example, a diversity receiver of the RAKE principle has a multitude of reception branches each of which is capable of synchronizing into a different signal component of the diversity signal. The receiver is therefore capable of receiving several signals simultaneously. Particularly the CDMA system utilizes RAKE receivers.
In diversity reception, the most typical diversity receivers combine the signals prior to or following the detection, and they comprise for example Selective combining, Maximal-ratio combining, and Equal-gain combining. The diversity signals are normally detected by using the Viterbi detection, whereby the signal combining takes place after the detection because the Viterbi detection produces the SNR estimate required. However, the most advantageous way is to combine the signals prior to the detection because this results in a higher signal gain. However, a diversity branch receiving no signal is not to be used in the combining. This makes the correct allocation of RAKE branches important. A closer description on diversity receivers is given for example in William C. Y. Lee: Mobile Communications Engineering, Chapter 10, Combining Technology, pp. 291-336, McGraw-Hill, USA, 1982, which is incorporated herein as reference.